(a) Field of the Invention
The present invention relates to a fibrous electrically-conductive filler and a method for preparing the filler. More specifically, the present invention relates to a fibrous electrically-conductive filler which is excellent in whiteness, non-toxic and cheap, which has electrical conductivity stable to temperature and humidity changes, which has a low volume resistivity, which is excellent in a electrical conductivity-imparting effect per unit weight and which can accordingly be incorporated into various basic materials such as paper, plastics, rubbers, resins, fibers and paints and varnishes for imparting electrical conductivity to these basic materials as well as a method for preparing the fibrous electrically-conductive filler.
(b) Description of the Prior Art
Electrical insulating properties of plastics become causes of various technical problems in several applications thereof. In particular, the electrical insulating properties of plastics become obstacles in, for instance, shielding electric parts from a relatively high electromagnetic field observed when a computer housing is, for instance, used or in discharging of electrically-charged parts. In addition, the electrical insulating properties of plastics likewise become causes of various problems in storing high performance explosives or IC parts, in preparing carpets which are subjected to an antistatic treatment or rubber products for medical use, or in preparing electrically-conductive adhesives for metals.
It has been known that a polymer can be converted into a electrically-conductive material through incorporation of electrically-conductive particles. As finely pulverized substances capable of imparting electrical conductivity to various basic materials such as plastics, and paints and varnishes through incorporation thereof into these basic materials, there have been known, for instance, metal particles or carbon black particles; particles of semiconductor oxides such as zinc oxide and iodides; tin oxide powder doped with, for instance, antimony or fluorine; zinc oxide powder doped with, for instance, aluminum or powder of, for instance, titanium oxide and aluminum oxide coated with tin oxide; and fibrous materials such as glass fibers, alkali metal titanate fibers and titanium oxide fibers coated with tin oxide.
If metal particles or carbon black particles are used as electrical conductivity-imparting substances, polymers are blackened through the addition of such additives. This is often undesirable in most of applications thereof. For instance, the use of zinc oxide particles suffers from a problem in that the resulting polymer shows fluctuation of the electrical conductivity due to changes in temperature and humidity. Moreover, antimony-doped tin oxide powder is excellent in a electrical conductivity-imparting ability, but the resulting polymer has a blue-black color tone due to the presence of antimony as a dopant and suffers from a problem of relatively low whiteness. Further the use of antimony as a dopant suffers from a problem of high toxicity. Thus, the polymers comprising antimony-doped tin oxide powder are substantially limited in the applications. For this reason, Japanese Un-examined Patent publication (hereinafter referred to as "J.P. KOKAI") No. Hei 4-154621 discloses a method for preparing titanium oxide powder coated with tin oxide as electrically-conductive powder free of antimony.
It has also been well-known that electrically-conductive fine substances incorporated into basic materials such as plastics and paints and varnishes must come in close contact with one another in these basic materials for imparting good electrical conductivity thereto and, therefore, if electrically-conductive spherical particles are used, a large quantity thereof must be incorporated in these basic materials. Moreover, if electrically-conductive powder used is expensive, the product obtained through incorporation thereof is limited in the applications from the viewpoint of the production cost.
To eliminate this problem, there have been proposed, for instance, the foregoing fibrous materials which have aspect ratios substantially higher than that of the spherical electrically-conductive particles and correspondingly high probability that the materials come in contact with one another when they are used in relatively small amounts. For instance, J.P. KOKAI Nos. Sho 59-102820 and Sho 62-59528 disclose electrically-conductive alkali metal titanate fibers obtained by coating fibers of alkali metal titanates represented by the formula: M.sub.2 O.nTiO.sub.2.mH.sub.2 O with tin oxide compounds as well as methods for preparing the fibers. In addition, J.P. KOKAI No. Sho 62-122005 discloses a method for preparing a white fibrous electrically-conductive filler which comprises treating alkali metal titanate fibers with an acid to liquate out the alkali components thereof and then coating the fibers with a tin oxide compound.
When these conventional fibrous electrically-conductive alkali metal titanate fillers are incorporated into basic materials such as plastics, rubbers, fibers and paints and varnishes to make them electrically electrically-conductive, they do not show any reduction in the electrical conductivity-imparting effect due to peeling off of the electrically-conductive substances from the fibrous materials, but suffer from a problem in that the alkali metal titanate fibers per se have low resistance to acids and alkalis as compared with those obtained by coating glass fibers with electrically-conductive substances. Moreover, they are easily damaged through breakage due to liquation of the alkali components during coating the electrically-conductive substances, and are micronized and made porous. This in turn increases the surface areas thereof and accordingly, the use of a large amount of a electrically-conductive substance is required for forming a electrically-conductive layer having an identical film thickness.
In general, the electrical conductivity of a product obtained through incorporation of a electrically-conductive filler is greatly dependent upon the amount (% by volume) of the electrically-conductive filler to be incorporated. Thus, the smaller the specific gravity of the electrically-conductive filler used, the smaller the amount thereof to be used for ensuring a desired electrical conductivity. In this respect, however, the foregoing alkali metal titanate fibers are not necessarily satisfied. Moreover, these alkali metal titanate fibers suffer from a problem of high toxicity since they have coated electrically-conductive layers comprising tin oxide doped with antimony.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide a fibrous electrically-conductive filler which can eliminate the foregoing drawbacks associated with the conventional techniques, more specifically a fibrous electrically-conductive filler which is excellent in whiteness, non-toxic and cheap, which has electrical conductivity stable to temperature and humidity changes and a low volume resistivity, which is excellent in a electrical conductivity-imparting effect per unit weight and which can accordingly be incorporated into various basic materials such as paper, plastics, rubbers, resins, fibers and paints and varnishes for imparting electrical conductivity to these basic materials.
Another object of the present invention is to provide a method for preparing such a fibrous electrically-conductive filler.
Under such circumstances, the inventors of this invention have conducted various studies to obtain a fibrous electrically-conductive filler free of the foregoing drawbacks, i.e.,
(1) a fibrous electrically-conductive filler having a low volume resistivity and ensuring a quite excellent electrical conductivity-imparting effect per unit weight (or in a small added amount) upon addition thereof to basic materials such as resins; PA1 (2) a fibrous electrically-conductive filler which does not show any loss of electrical conductivity due to peeling off of a electrically-conductive film applied onto the surface of the filler during incorporating into basic materials such as paper, plastics, rubbers, resins, fibers, and paints and varnishes; PA1 (3) a fibrous electrically-conductive filler which is white or transparent in itself and, accordingly capable of imparting any desired color tone to basic materials such as paper, plastics, rubbers, resins, fibers, and paints and varnishes, to which the electrically-conductive filler is incorporated, through simultaneous or separate addition of a coloring agent; PA1 (4) a fibrous electrically-conductive filler being free of any harmful substance such as antimony, have found out that it is effective to use, as a core material, fibrous aluminum borate having high strength, high elasticity modulus, good heat resistance, good resistance to chemicals and a low true specific gravity as compared to those for the conventional alkali metal titanate fibers and have thus completed the present invention.
According to an aspect of the present invention, there is provided a fibrous electrically-conductive filler which has a light weight and excellent electrical electrical conductivity and comprises fibrous aluminum borate as a core material provided thereon with a tin oxide layer; or fibrous aluminum borate as a core material provided thereon with, in order, a titanium oxide layer and a tin oxide layer wherein the tin oxide layer is free of antimony.
According to another aspect of the present invention, there is provided a method for preparing a fibrous electrically-conductive filler comprising the steps of forming a coating layer of tin oxide hydrate on the surface of fibrous aluminum borate and then calcining the coated fibrous aluminum borate at a temperature ranging from 150.degree. to 600.degree. C. in an inert gas atmosphere or a reducing atmosphere to form a tin oxide layer; or forming a titanium oxide layer on the surface of fibrous aluminum borate, then forming a coating layer of tin oxide hydrate on the surface of the titanium oxide layer and then calcining the coated fibrous aluminum borate at a temperature ranging from 150.degree. to 600.degree. C. in an inert gas atmosphere or a reducing atmosphere to form a tin oxide layer.
The fibrous electrically-conductive filler according to the present invention has a low volume resistivity and an excellent electrical conductivity-imparting effect per unit weight upon incorporation into basic materials such as resins or paints and varnishes. Further the fibrous electrically-conductive filler is excellent in whiteness and transparency in itself. Therefore, any color tone may be imparted to these basic materials through simultaneous incorporation of the filler and coloring agents. Moreover, the filler is non-toxic since it is free of any harmful substances such as antimony. The fibrous electrically-conductive filler of the invention thus has high quality and permits the reduction of the price.